The present invention is directed to ducts and duct liners and to a method of forming ducts and duct liners from rigid and semi-rigid insulation boards wherein the configuration of the kerfs formed in the insulation boards as well as the depths, widths and spacing of the kerfs in the insulation boards, control the flexibility of the insulation boards and the structural compression characteristics of the kerf sidewalls or shoulders to facilitate the formation of ducts and duct liners of desired diameters from the insulation boards.
Heating, ventilating and air conditioning systems, exhaust systems and other similar gas flow systems found in large building structures, such as, but not limited to convention centers, sports domes, warehouses, factories and the like require large diameter, high capacity, insulated round or flat oval air ducts for conveying conditioned air from air conditioning units to locations throughout the building structure and for returning air to the air conditioning units for heating or cooling and recirculation or for conveying other gases such as exhaust gases. These large diameter, insulated round or flat oval air ducts typically range from about twenty inches to over one hundred inches in diameter.
Presently, the most commonly used ducts for such large, high capacity, air and gas flow systems are double wall insulated metal ducts. These large diameter, high capacity, metal ducts for conveying air and other gases comprise an outer tubular metal shell, an inner perforated tubular metal shell and one or more layers of glass fiber insulation blankets or other insulation materials intermediate the outer tubular shell and the inner tubular shell. These ducts normally have either a round or a flat oval transverse cross section. Due to their construction which, in essence, comprises a first tubular metal duct contained within a second tubular metal duct, and the labor required to both assemble the first metal duct within the second metal duct and insulate the space between the ducts while maintaining the ducts concentric with respect to each other, these double wall, insulated metal ducts for conveying gases, such as conditioned air, return air, exhaust gases and the like, are relatively expensive.
A second duct, currently in use for conveying gases, comprises a round, tubular metal shell internally lined with a round tubular, preformed, self-supporting, molded, glass fiber insulation sold by Schuller International Inc., under the trademark xe2x80x9cSPIRACOUSTICxe2x80x9d. This duct system exhibits good thermal and acoustical insulating properties and eliminates the need for a costly perforated metal liner. However, because of manufacturing restrictions relating to the molding of the glass fiber insulation into its tubular shape, this duct system has been size restricted for use with tubular metal shells having internal diameters of twenty-six inches or less.
Thus, although the molded glass fiber insulation liners provide a cost effective lining system with good performance characteristics for tubular metal ducts having internal diameters of twenty-six inches or less, there has been a need to provide large diameter air and gas handling ducts with an insulated lining system other than the costly perforated metal lining system discussed above.
The ducts and duct liners of the present invention comprise rigid or semi-rigid insulation boards (commonly referred to as xe2x80x9cduct boardsxe2x80x9d) which have a plurality of parallel kerfs (narrow grooves) in their gas stream surfaces that enable the insulation boards to be curved or bent about an axis parallel to the kerfs into tubular shapes to form ducts or duct liners. Typically, the insulation boards are made of a fibrous insulation, a cellular foam insulation or a fibrous and cellular foam insulation laminate. The insulation boards are typically four feet by ten feet or eight feet by ten feet and from about one inch to about four inches thick. For the purposes of this application the dimension of the insulation board parallel to the kerfs is defined as the length of the insulation board and the dimension of the insulation board perpendicular to the kerfs is defined as the width of the insulation boards.
Where the duct or duct liner being formed has a circumference equal to or less than the width of a kerfed insulation board, the insulation board or a portion thereof is bent and formed into a tube of the desired diameter with the abutting lateral edges of the insulation board preferably being taped or otherwise held together. As a duct liner, the duct is then inserted into a tubular metal shell having internal dimensions equal to or slightly less than the external dimensions of the duct liner. Where the duct or duct liner being formed has a circumference greater than the width of a kerfed insulation board, the insulation board is taped or otherwise affixed to a second insulation board or portion thereof and the joined insulation boards are bent and formed into a tube of the desired diameter which can be inserted into a tubular metal shell.
The gas stream surfaces of the insulation boards may be treated or untreated. However, with respect to fibrous insulation boards, preferably, the gas stream surfaces of the insulation boards are provided with a coating, such as a polymeric coating, or are provided with a facing to encapsulate fibers and/or particles of the insulation boards within the insulation boards.
The kerfs formed in the gas stream surfaces of the insulation boards, to enable the insulation boards to be bent into a tubular shape, are sized so that the shoulders of the kerfs adjoining the air stream surfaces of the insulation boards close together and are subjected to compression when the insulation boards are bent into a tubular shape of the desired dimensions for the duct or duct liner being fabricated. With the insulation material of the insulation boards in compression at the shoulders of the kerfs, the insulation boards retain the tubular shape into which the insulation boards have been formed. If the kerfs are to wide, the shoulders of the kerfs adjoining the gas stream surfaces of the insulation boards are not subjected to compression when the insulation boards are bent and shaped into a tubular configuration of the desired dimensions and the duct or duct liners formed from the insulation boards do not retain the desired tubular shape. Furthermore, since the kerfs formed in the insulation panels of the ducts or duct liners of the present invention close together at the gas stream surface of the ducts or duct liners, fibers or dust in the kerf sidewalls are not exposed to the interior of the ducts or duct liners and the air or other gas streams being conveyed by the ducts.
In one preferred embodiment of the present invention, the narrow kerfs have straight sidewalls extending perpendicular to the gas stream surfaces of the insulation boards with shoulders adjoining the gas stream surface which close when the insulation boards are bent and curved into the desired tubular shape. While the sidewalls of these kerfs meet at the gas stream surfaces of the insulation boards to place the shoulders of the kerfs adjoining the gas stream surface of the duct in compression and to seal off the kerfs, a series of air spaces, having triangular cross sections, are formed in the walls of the ducts or duct liners formed from the insulation boards. These air spaces reduce the insulating properties of the ducts or duct liners formed from the insulation boards and when using kerfs with straight sidewalls which extend perpendicular to the gas stream surfaces of the insulation boards, the sizes of these air spaces increase as the thicknesses of the insulation boards used to form the ducts or duct liners are increased.
Vee-shaped kerfs or substantially vee-shaped kerfs can eliminate or substantially eliminate the air spaces within the walls of the ducts formed from the kerfed insulation boards of the present invention and, to assure optimum insulating characteristics in the ducts or duct liners formed from the kerfed insulation boards, vee-shaped kerfs or substantially vee-shaped kerfs should be used in insulation boards two inches thick or greater. However, if the vee-shaped kerfs are too wide at the gas stream surfaces of the insulation boards, the shoulders of the kerfs adjoining the gas stream surfaces of the insulation boards do not close together and are not subjected to compression when the insulation boards are formed into a tubular shape. As discussed above, the insulation material in the shoulders of the kerfs and preferably, extending for the entire or substantially the entire depth of the sidewalls, should be subjected to compression so that the insulation boards will retain the tubular shape into which the insulation boards have been formed and the shoulders should be closed together so that fibers and/or particles in the kerf sidewall surfaces are not exposed to the interior of the ducts and the high velocity gas streams being conveyed by the ducts. Thus, the kerfs formed in the gas stream surfaces of the insulation boards in accordance with a preferred embodiment of the present invention have vee-shaped or substantially vee-shaped cross sections with the widths of the kerfs at the gas stream surfaces of the insulation boards being controlled so that the air spaces in the walls of the ducts or duct liners formed from the insulation boards are eliminated or substantially eliminated and the shoulders of the kerfs adjoining the gas stream surfaces of the insulation boards and, preferably, the entire or substantially the entire remaining portions of the kerf sidewalls are placed under compression when the insulation boards are formed into the duct or duct liner of preselected dimensions.
The flexibility of the insulation boards, in the direction of the widths of the insulation boards, may be controlled by the depth, width and/or spacing of the kerfs formed in the gas stream surface of the insulation boards. An insulation board with kerfs of a certain depth and width that are spaced from each other a certain spacing will exhibit a certain flexibility. To make the insulation boards more flexible, the depths of the kerfs can be increased, the widths of the kerfs can be increased and/or the spacing between kerfs can be decreased. To make the insulation boards less flexible, the depths of the kerfs can be decreased, the widths of the kerfs can be decreased and/or the spacing between the kerfs can be increased.